Preliminary comparisons of the activities of several enzymes in liver and blood components of control and cancer patients have indicated substantial quantitative changes in the activities of enzymes in liver (arginase, peptidyl proline hydroxylase, alanine amino-transferase, hexokinase, glucose-6-phosphate dehydrogenase, thyimidine kinase (particulate), pyrroline-5-carboxylate reductase (cold Stable), and glutamate dehydrogenase (soluble)) and in blood plasma (gamma-glutamyl transferase, arginase), erythrocytes gamma-glytamyl transferase, arginase, pyrroline-5-carboxylate reductase), and leukocytes (gamma-glutamyl transferase, arginase) in cases of clinically diagnosed cancer. Abnormally high actvities of gamma-glutamyl transferase in liver and blood appear to be associated with cancer of the pancreas while raised arginase and plasma glutamate dehyrogenase levels may distinguish patients with hepatic malignancies from all others. Although many more analyses on a wider spectrum of controls and cancer patients will be required before the significance of these preliminary observations can be assessed, the tentative results suggest that tumor marker enzymes may be identifiable in human blood and quantitative determinations of their activities may eventually contribute a non-invasive technique for screening and monitoring patients. Extensions of preliminary studies of enzyme activities of human tissues (fetal and adult) have confirmed our hypothesis that the enzyme composition of host tissues (liver and lung) represents an intemediate stage between fetal and adult congener, reflecting more fetal characteristics than the normal adult and fewer than tumors themselves. Quantitative enzyme abnormalities in morphologically normal host tissues (liver and lung not involved with neoplastic growth) may be detected easily by available methods and may offer a refinement in the diagnosis of and prognosis for cancer patients.